1. Field of the Invention
The present invention relates to a process for producing a toner for development of electrostatic images.
2. Description of Related Art
To increase printing speed and achieve further saving of energy for reduction of environmental loads in electrophotographic image forming apparatuses, a toner for development of electrostatic images (hereinafter, simply referred to as “toner”) which is capable of heat fixing at lower temperatures has been recently required. Such a toner needs lowering of the melting temperature and melt viscosity of a binder resin, and a toner having low-temperature fixability enhanced by adding a crystalline resin such as a crystalline polyester resin is suggested (e.g., see Japanese Patent Application Laid-Open No. 2001-222138). When a toner containing a crystalline resin is heated to a temperature higher than or equal to the melting point of the crystalline resin in production of the toner, however, the crystalline resin becomes compatible with an amorphous resin even in production, which inconveniently deteriorates the high-temperature storability.
Annealing (hereinafter, also referred to as heat treatment) is known as means for enhancing the high-temperature storability of a toner having such a configuration. For example, it is reported that heat treatment at a temperature higher than or equal to the glass transition temperature of an amorphous resin and lower than or equal to the melting point of a crystalline resin−10° C. for a long duration allows the crystalline resin which is compatible with the amorphous resin to recrystallize to enhance the high-temperature storability (e.g., see Japanese Patent Application Laid-Open No. 2009-063992).
In addition, there is known a method of controlling the heating/retention temperature for an aqueous dispersion of a crystalline resin of a block polymer (e.g., see Japanese Patent Application Laid-Open No. 2014-211632). According to the document, this method enables control of the crystalline resin domain even in recrystallization of the crystalline resin, and thus the crystalline resin domain can be finely dispersed to prevent deterioration of fixability.
Further, it is reported that heating and retention of a toner composition containing a binder resin containing a crystalline polyester under predetermined conditions allows the toner to keep the low-temperature fixability and high-temperature storability for a long period (e.g., see Japanese Patent Application Laid-Open No. 2012-42508). Furthermore, a method is known in which a differential scanning colorimetry (DSC) curve is obtained in measurement for a crystalline polyester resin by using a differential scanning colorimeter and heat treatment is performed at the onset temperature of an endothermic peak in the DSC curve ±5° C. (e.g., see Japanese Patent Application Laid-Open No. 2012-98697). Moreover, there is known heat treatment of a toner particle containing a crystalline resin and an amorphous resin at a temperature which is higher than or equal to the glass transition temperature of the crystalline resin and is the recrystallization temperature ±10° C. (e.g., see U.S. Pat. No. 7,494,757).
In dry heat treatment, however, the elevation of the glass transition temperature, the increase of the domain diameter of a crystalline resin in a toner, etc., are caused due to the change of the moisture adsorption state of a toner, which complicates development of low-temperature fixing performance at a level required in recent years. In addition, a crystalline resin exposed in the surface of a toner lowers the surface resistance of the toner and deteriorates the charging characteristics of the toner, which may inconveniently cause troubles such as toner splashing. In heat treatment in an aqueous medium, a crystalline resin may have been exposed in the surface of a toner.
Thus, conventional production processes for a toner leave room for improvement from the viewpoint of further enhancement of the low-temperature fixability of a toner and reduction of toner splashing in actual printing for a long period.